1. Field of the Invention
The present invention relates to a measurement apparatus, an exposure apparatus, and a device fabrication method.
2. Description of the Related Art
An exposure apparatus projects and transfers a pattern formed on a reticle (mask) onto a substrate such as a wafer via a projection optical system. The exposure apparatus measures the surface position of a substrate at a predetermined position on the substrate using a surface shape (surface position) measurement unit of the light oblique-incidence system during exposure (or before exposure), and performs correction to align the substrate surface with an optimum imaging position prior to exposure of the substrate at the predetermined position. In particular, a scanner measures not only the surface position level (focus) of a substrate but also the surface tilt of the substrate in the longitudinal direction (i.e., a direction perpendicular to the scanning direction) of the exposure slit.
Japanese Patent Laid-Open No. 6-260391, U.S. Pat. No. 6,249,351, and PCT(WO) 2006-514744 propose details of such focus and tilt measurement techniques. Japanese Patent Laid-Open No. 6-260391 and U.S. Pat. No. 6,249,351, for example, disclose techniques using optical sensors. PCT(WO) 2006-514744 discloses a technique using a gas gauge sensor which measures the surface position of a substrate by blowing air onto the substrate. Moreover, a technique using a capacitance sensor is proposed.
In recent years, as the wavelength of the exposure light shortens and the NA of the projection optical system increases, the depth of focus extremely decreases. To keep up with this trend, the accuracy of aligning the surface of a substrate to be exposed with an optimum imaging position, that is, the so-called focus accuracy is increasingly becoming stricter. Under the circumstance, one technique for improving the measurement accuracy is attracting a great deal of attention. This technique measures the surface shape (surface position) of a substrate based on an interference pattern (interference signal) formed by interference between light (measurement light) from the substrate surface (measurement target surface) and light (reference light) from a reference surface.
In this technique, light which has a broad wavelength bandwidth and is emitted by a light source is split into two light beams, one light beam enters the measurement target surface, and the other light beam obliquely enters the reference surface. Then, the measurement light reflected by the measurement target surface and the reference light reflected by the reference surface are combined to detect an interference pattern (interference signal) formed by interference between the measurement light and the reference light. An interference signal is detected while driving the measurement target surface in a predetermined direction (level (focus) direction), and the surface shape of the measurement target surface can be obtained based on a change in the detected interference signal.
Techniques of this kind can shorten the coherence length using light with a broad wavelength bandwidth, thereby setting a measurement range wider than that which can be set using monochromatic light. In addition, these techniques can advantageously reduce interference signal errors attributed to a resist (photosensitive agent) applied on the substrate.
Unfortunately, in the prior arts, when the output from the light source fluctuates with time, noise (light amount noise) that mixes in the interference signal increases, so the surface shape measurement accuracy and reproducibility deteriorate. Because the interference signal can be obtained within a certain finite time range, a fluctuation in output from the light source within that time range, in turn, generates a fluctuation in the amount of light at each measurement position (each driving position on the measurement target surface), and the amount of light naturally differs for each measurement point. Thus, the accuracy of obtaining the peak position of the interference signal deteriorates and, eventually, the surface shape measurement accuracy and reproducibility deteriorate.